This application claims the benefit of Korean Patent Application No. 1999-10397, filed on Mar. 25, 1999, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to an X-ray image detector, and more particularly to an X-ray image detector fabricated utilizing a Thin Film Transistor (TFT) array process and a method for fabricating the same.
2. Description of Related Art
The widely used X-ray detection method for medical diagnosis is such that an X-ray detecting film is used to produce a photograph and some predetermined printing procedure is required to obtain the result.
However, digital X-ray detectors employing TFTs have been developed recently due to the development of semiconductor technology. This X-ray image detector has an advantage that a real time result for diagnosis can be obtained immediately after photographing because it uses a TFT as a switch.
The above-described X-ray image detector and a TFT-LCD have many characteristics in common in Table 1.
As shown in Table 1, the opening ratio of an X-ray image detector is larger than that of a TFT-LCD by 15 to 25%. This is because the metal line of a TFT-LCD is covered with black matrix to prevent the deterioration of picture quality. However, in an X-ray image detector, as the total surface of the ITO pixel electrode corresponds to the opening part, its opening ratio is superior to that of a TFT-LCD.
FIG. 1 is a schematic cross-sectional view illustrating the structure and operation of X-ray image detector 100 which comprises lower substrate 1, thin film transistor 3, storage capacitor 10, pixel electrode 12, photoconductive film 2, protection film 20, conductive electrode 24 and high voltage D.C. (direct current) power supply 26.
Photoconductive film 2 produces internal electric signals, i.e. pairs of electron (e) and holes (h), in proportion to the strength of external signals such as incident electromagnetic waves or magnetic waves. The photoconductive film 2 serves as a conversion to detect external signals, particularly X-rays, and convert them to electric signals. Electron-hole pairs (6) are gathered in the form of electric charges at pixel electrode 12 located beneath the photoconductive film 2 by a voltage (Ev) applied to conductive electrode 24 by the high voltage D.C. power supply 26, and then is stored in storage capacitor 10 formed in connection with a common electrode grounded externally. Charges stored in the storage capacitor 10 are transferred by TFT 3, controlled externally, to an external image display device and forms X-ray images.
In an X-ray image detector, to detect and convert even a weak X-ray signal into electric charges, it is required to decrease the number of electric charges trapped in the photoconductive film 2, and to decrease the current in non-vertical directions by means such as applying a high voltage (more than 10V/xcexcm) in the vertical directions between conductive electrode 24 and pixel electrode 12.
Electric charges in the photoconductive film 2 produced by X-ray energy are trapped and gathered on a protection film (not illustrated),which protects the channel part of the TFT 3, as well as on the pixel electrode. This electric charges trapped and gathered induce electric charge into the channel region in the upper part of TFT 3, producing a high leakage current even when TFT 3 is in an xe2x80x9coffxe2x80x9d state, thus inhibiting switching operation of TFT 3.
Moreover, electric signals stored in the storage capacitor 10 are discharged externally due to the high leakage current in the xe2x80x9coffxe2x80x9d state, as a result of which, the desired image can not be obtained.
FIG. 2 is a cross-sectional view schematically illustrating a conventional X-ray image detector. U.S. Pat. No. 5,498,880 discloses one example of this kind of structure wherein pixel electrode 12 is extended to cover the upper part of TFT 3, (so called xe2x80x9cmushroom structurexe2x80x9d) to prevent the trapping of electric charges on the upper part of TFT 3, induced from the electric charges produced in photoconductive film 2 by X-ray.
The manufacture of the conventional X-ray image detector will be described hereinafter referring to FIG. 2.
First, substrate 1 is deposited with a metal and patterned to form a gate electrode 31. Then, SiNx is deposited thereon in the thickness of about 100 nm to form a first insulation film 34a. After the formation of film 34a, a transparent conductive material is deposited and patterned to form a first storage electrode 40. ITO (indium tin oxide) is most commonly used as the transparent conductive material.
After forming the first storage electrode 40, second insulation film 34b is formed on the first insulation film 34a and first storage electrode 40. At a predetermined position of the second insulation film 34b on the first storage electrode 40, a contact hole 41 is formed for contact with a ground line 42 that will be formed later.
Thereafter, a source/drain metal material is deposited and patterned to form a source electrode 33, drain electrode 32 and ground line 42. The source/drain metal is usually aluminum which has a low resistance and good deposition properties. Protection layer 46 is formed after the formation of 33, 32 and 42, in order to protect TFT 3.
In the protection layer 46 contact holes are formed on the source electrode 33 for contact with a second storage electrode which will be formed later. Then, the protection layer 46 formed in the upper part of the first storage electrode 40 is etched out except on ground lines 42 for increasing the capacity of the storage capacitors. In the described structure, the practical charging capacity corresponds to the portion labeled Cst in FIG. 2. Now, ITO is deposited and patterned to form a second storage electrode 12 which serves as a pixel electrode, and a photoconductive film 2 is formed by deposition on the overall substrate 1. The later procedures are abbreviated here.
In an X-ray image detector adopting the so-called xe2x80x9cmushroom structurexe2x80x9d as described above, as electric charges produced by X-ray energy gather on the pixel electrode 12 of the storage capacitor, the electric potential of the storage capacitor increases. The increased potential causes an increased capacity of a parasitic capacitor which is formed between the xe2x80x9cmushroomxe2x80x9d associated with the pixel electrode and TFT 3.
The capacity of a parasitic capacitor is inversely related to the thickness of the protection layer 46 to protect the channel part in the upper part of TFT 3 such that the capacitance increases as the thickness of the protection layer 46 is reduced, inducing a large amount of charges to the channel part, which increases the amount of leakage current even if TFT is in an xe2x80x9coffxe2x80x9d state and deteriorates its switching operation.
Though the capacity of a parasitic capacitor of TFT 3 may be decreased if the thickness of an organic insulation film used as dielectric material of storage capacitor is increased to more than 1.5 xcexcm, the rear surface of TFT 3 under an acrylic protection film of the conventional X-ray image detector can have about 1 xcexcm thickness, which allows increase of leakage current in an xe2x80x9coffxe2x80x9d state.
Furthermore, second insulation film 34b used as a dielectric of the storage capacitor is formed to be thin with a thickness of about 200 nm in a conventional X-ray image detector 50, the second insulation film may be etched out (overetching) while etching the organic insulation film as protection layer 46. This causes an electrical short between the first and second storage electrodes and accordingly the number of point defects is increased, causing a low yield.
Moreover, because the protection layer 46 in the true storage capacitor is substantially thick, the Cst portion is relatively small. To solve the problem, the protection layer 46 is partially etched to form the Cst, but a satisfactory result has not been obtained because there is a limit in extending the Cst by utilizing only part of a total pixel.
An object of the present invention is to provide an X-ray image detector in which the xe2x80x9coffxe2x80x9d electric current is decreased by lowering the leakage current of a TFT.
Another object of the present invention is to provide a manufacturing method of an X-ray image detector with high yield by reducing the instances of electrical shorts between gate and source/drain electrodes and storage electrode.
A further object of the present invention is to provide an X-ray image detector with a storage capacitor having an increased capacity.
To achieve the objects, the present invention provides, in one aspect, an X-ray image detector which comprises: a photoelectric conversion part producing electric charges in accordance with a received amount of light; a charge storage part (storage capacitor) comprising a first storage electrode, a dielectric layer deposited on the first storage electrode, a second storage electrode on the dielectric layer, and a pixel electrode in contact with total surface of the second storage electrode and collecting the electric charges produced in the photoelectric conversion part; and a switching part controlling the release of the electric charges stored in the storage capacitor.
In another aspect, the present invention also provides an X-ray image detector which comprises a substrate; a gate electrode formed on the substrate; a first insulation film formed on the substrate and gate electrode; a first storage electrode formed on a predetermined position of the first insulation film; an active layer formed the first insulation film on the gate electrode; a source and drain electrode formed on the active layer; a second insulation film formed on the source and drain electrode and the first storage electrode and having a source contact hole exposing a portion of the source electrode; a second storage electrode formed on the second insulation film; a third insulation film covering a residual part except the second storage electrode and source contact hole; and a pixel electrode formed on the exposed source electrode, second storage electrode and third insulation film.
In another aspect, the present invention also provides a method for fabricating an X-ray image detector, comprising forming a gate electrode on a substrate; forming a first insulation film on the substrate and gate electrode; forming an active layer on a first insulation film on the gate electrode; forming a source electrode and drain electrode on the active layer and forming a ground line at a predetermined position on the first insulation film; forming a first storage electrode which covers the ground line; forming a second insulation film on the first storage electrode, source electrode and drain electrode; forming a second storage electrode on the second insulation film on the first storage electrode; depositing an organic insulation film covering the second insulation film and second storage electrode; patterning the organic film and second insulation film so that a portion of the source electrode and the total second storage electrode may be exposed by etching; forming a pixel electrode in contact with the source electrode and second storage electrode, on the organic insulation film remained after etching; forming a photoconductive film on the pixel electrode; and forming a conductive electrode on the photoconductive film.